1. Field of the Invention
The present invention relates to a digital communications system for discriminating a start position of a transmission format on a receiving side.
2. Description of the Related Art
In some digital communications systems, data transmission is executed by arbitrarily selecting a transmission format from a plurality of transmission format candidates. In such a system, identification information of an employed transmission format is indicated to a receiving side by a transmitting side in some cases or often is not in the other cases.
In the latter cases, since the employed transmission format is not known to the receiving side, the transmission format needs to be detected with reference to a reception signal. This detection is generally called blind detection and is often executed in a receiving device of a W-CDMA system (cf., for example, 3GPP TS 25.212 V5.10.0 (2005-06) Annex A (informative): Blind transport format detection).
In 3GPP TS 25.212 V5.10.0 (2005-06) Annex A (informative): Blind transport format detection), a method of blind detection for a plurality of formats of different transmission data sizes is disclosed. A.1.2. Blind transport format detection using CRC that is disclosed therein executes error detection using CRC at a position corresponding to each of the formats while executing error-correcting decoding of a convolutional code and detects that a format in which correct CRC is detected, that is, no error is found is a transmission format.
However, since decoding of the error-correcting code needs to be started at the start position of the transmission format, this method can be applied only to a case where the start position of the transmission format is constant and the data size is varied according to the transmission format, but not to a case where there are a plurality of start positions of the transmission format.
An actual communications system may have a plurality of candidates for the start position of the transmission format. For example, if a set of transmission information is split into a plurality of frames and transmitted, it is unknown on the receiving side which frame of the information is first transmitted. Such a case is seen in an initial search of a cellular system.
In this case, the receiving side must execute error-correcting decoding for each of a plurality of candidates for the transmission formats that start at an unknown frame timing. For this reason, if blind detection of the transmission format is executed, processing a large amount of data is needed.
In short, the conventional digital communications system has a problem that if an unknown transmission format is subjected to blind detection and the transmission format has a plurality of candidates for the start position, processing a large amount of data is needed to detect the start position.